The Public Employees Federation, with 55,000 members, bucked Cuomo after supporting him during his first election bid in 2010. Public unions have been at odds with Cuomo over contentious contract negotiations during his first year in office.

"PEF is a very democratic union and today is a testament of that," PEF president Susan Kent said in a statement.

The endorsement came the same day that another union, the New York State United Teachers, declined to endorse a candidate in the governor's race.

Andrew Pallotta, NYSUT's executive vice president, said the union couldn't come to a consensus among the candidates.

"I think the only peaceful place that we could find was a no endorsement in the governor's race," he said.

NYSUT, which has 600,000 members, endorsed candidates for Congress and the state Legislature and backed incumbent Attorney General Eric Schneiderman and Comptroller Thomas DiNapoli, both Democrats.

The PEF boost will help Teachout, a Fordham University law professor, in her underdog bid against Cuomo in a Sept. 9 primary.

Cuomo's campaign is trying to knock her off the ballot by challenging her residency in court. A state Supreme Court judge ruled Monday that Teachout had lived in New York the required five years to be on the ballot, but Cuomo's lawyers are appealing.

"The Public Employees Federation endorsement is an act of courage and bravery by workers who have a lot at stake," Mike Boland, Teachout's campaign manager, said in a statement. "Today, they rose above the culture of fear that is pervasive in Albany under Gov. Cuomo. The governor and his old boys club better watch out, this challenge is serious and it isn't going away."

NYSUT has battled the Democratic governor over teacher evaluations and the tougher testing standards known as Common Core. Cuomo and the Legislature did heed to some of the concerns of the union, parents and school officials earlier this year, delaying the use of the test results on students' records and delaying the use of the test on most teachers' evaluations.

But NYSUT president Karen Magee, the former Harrison Association of Teachers president in Westchester County, indicated after her election in April that the union would not likely back Cuomo in his re-election bid against Republican Rob Astorino, the Westchester County executive.

Activist Randy Credico is also on the Democratic primary ballot.

NYSUT had $646,706 in his political action committee. The union also has an extensive operation to draw out supporters to the polls, as does PEF.